This invention relates generally to microwave oscillators and in particular to low-noise microwave oscillator circuits.
It is well known that many advanced electronic systems for communication and navigation require circuits, including oscillator circuits, having low noise for low noise applications. Such low noise applications include space communications, electronic warfare counter measures and modern radar techniques. In a microwave oscillator, frequency modulation (FM) noise is a problem due to the noise characteristics of the components in the oscillator. In many applications, it is advantageous to reduce the FM noise characteristics of the oscillator. One technique known for lowering the FM noise of a single resonator feedback oscillator including an amplifier with a gallium arsenide field effect transistor (FET) as an element is providing a high Q element in the feedback circuit as described in an article by O. Ishihara, T. Mori, H. Sawano and M. Nakatani entitled "A Highly Stabilized GaAs FET Oscillator Using a Dielectric Resonator Feedback Circuit in 9-14 GH.sub.z " IEEE Transactions on Microwave Theory and Techniques, vol. MTT-28, No. 8, Aug. 1980, pp. 817-824. However, the FM noise, although reduced, is still relatively high.
A second technique known for reducing noise is using a gallium arsenide FET as an active element in a single resonator (tunable or fixed-frequency) microwave oscillator as described in U.S. Pat. No. 4,555,678, issued Nov. 26, 1985, entitled "Microwave Oscillator" (which patent is assigned to the same assignee as this application). In said patent, it is described that a gate bias port of a FET amplifier can be used as the tuning port of the oscillator, since the gate-to-source capacitance of the FET is dependent on gate-to-source voltage. Thus, a signal applied between the gate and source terminals changes the value of the gate-to-source capacitance which, in turn, modulates the phase of the signal amplified by the FET. The dispersive phase response of the resonator in the feedback circuit converts the phase modulation of the amplified signal to a frequency modulated signal at the output of the oscillator. Thus, the frequency of the oscillator is modulated by the signal applied to the gate bias port serving as the tuning port of the oscillator.
Although it is useful in many applications, using the gate bias port of the FET amplifier as the tuning port of the oscillator has some disadvantages. For example, the gate-to-source capacitance versus voltage relationship is difficult to accurately predict because the relationship is a function of the large signal conditions under which the oscillator reaches steady state and the phase shift versus gate voltage characteristic of the amplifier is not necessarily monotonic. The relationship also changes from one FET to another because of variability in FET parameters from one FET to another so that it is very difficult to predict accurately the modulation sensitivity of the oscillator. Since modulation sensitivity is a critical frequency lock loop design parameter, the modulation sensitivity of every oscillator has to be measured individually and the parameters of each corresponding frequency lock loop have to be adjusted accordingly to obtain the desired performance of the frequency lock loop. Furthermore, this method of oscillator frequency tuning is not particularly well suited for the use of one-port devices (e.g., Gunn or IMPATT diodes) as the active element in the oscillator circuit. Finally, when the oscillator power supplies are first turned on, the frequency lock loop output voltage fed to the gate bias port may be excessive such that the effective gate DC bias voltage is positive causing large gate current conduction which can destroy the transistor.